The invention relates to a method for determining the viscosity of an operating liquid, such as the engine oil, of an internal combustion engine.
A method of this kind is known from DE 195 18 776 A1 which describes a method for establishing the viscosity of an operating liquid on the basis of the changes in the level of the operating liquid over a period of time. In this known method, the viscosity of the engine oil is determined after having turned off an internal combustion engine by measuring the variation of the engine oil level over a period of time. The return of the engine oil to the sump after the engine has been turned off takes place with a time delay that depends on the viscosity. Apart from the variation of the level of the oil against time, the oil temperature is also a factor in the known method because the viscosity of the engine oil generally depends on its temperature.
Apart from these variables that are to be measured in a simple manner, the measurement of the viscosity in accordance with the known method is, however, also influenced by other variables. For instance, a high dynamic loading of the internal combustion engine over a period of time immediately before it is turned off can lead to foaming of the engine oil. The consequent slower change in level after turning the engine off results in corruption of the actual measurement. An objective measured variable for the dynamic loading of the internal combustion engine cannot, however, be determined in a simple manner.
Furthermore, when the internal combustion engine is turned off while the vehicle is in an inclined position, this also has an adverse effect on the return flow of the engine oil into the sump.
Another disadvantage is due to the fact that the electronic modules needed for the measurement still have to be actively operated for the required period of time after the internal combustion engine has been turned off.
The object of the invention is to specify a method for determining the viscosity of an operating liquid of an internal combustion engine, whereby the method supplies a precise result with few measured variables and is performed while the internal combustion engine is in operation.
This object is achieved by a method according to the invention, having the following features.
In the method for determining the viscosity of an operating liquid of an internal combustion engine of a vehicle by determining the level of the operating liquid, a sensor is provided for establishing the level of the operating liquid, said sensor having a measuring chamber that is connected via a damping device to a reservoir of the operating liquid. During the starting operation of the internal combustion engine, the variation of the level (H) of the operating liquid in the measuring chamber of the sensor is measured. The viscosity is determined from the variation of the level (H) against time. As opposed to the methods known from DE 195 18 776 A1, a sensor is used here that has a structure which damps the measured value curve. Acquisition of the measured values at the time of starting the internal combustion engine has the decisive advantage that controlled marginal conditions regularly prevail.
In a first embodiment of the method, the temperature (T) of the operating liquid is measured during the starting operation and is accounted for in the determination of the viscosity. This allows the values obtained for the viscosity to be corrected at the start to allow for the influence of temperature.
In a second embodiment of the method, the rotational speed of the internal combustion engine is also measured after the starting operation and allowed for when determining the viscosity. This allows the values obtained for the viscosity to be corrected at the start to allow for the influence of the change in operating liquid volume resulting from the rotational speed.
In a further embodiment of the method, the temperature of the operating liquid during the starting operation and the variation of the temperature of the operating liquid against time are also taken into account when determining the viscosity.
The invention will now be described with reference to an example embodiment and the single accompanying drawing FIGURE.